The present invention is concerned with providing “knock down” cylindrical-conical structures such as water tanks and risers which can be assembled from rings to provide a desired length or height, and which are adaptable to having closures or lids. As is well known, when a generally tubular structure is constructed in such a knock down way, the costs of storage and shipping can be reduced.
A riser for a plastic or concrete septic tank used in wastewater treatment is one application for such structures. The purpose of the riser is provide a space which extends upwardly from the access port of a septic tank to, or near to, the surface of the soil in which the tank is buried. A riser is supposed to keep surface water and soil from entering the port of the tank. Thus the joints between rings should have seals or other means aimed at preventing the passage of water. A similar requirement is presented in connection with a hole in earth that provides access to a sewer line or other buried things by means of a manhole. A casing or liner, sometimes referred to as a chimney, surrounds the manhole opening to maintain the hole within the earth. When such casings are made of plastic segments connected end-to-end, they present the same essential needs attending risers. In the present description the term riser shall be construed to embrace structures which are of the nature of manhole casings.
The riser should have a minimum diameter which is no less than the diameter of the access port on the top of a septic tank, which commonly is of about 60 cm. In the past such risers have been provided either as a one piece structure, or as a multiplicity of circular rings which are commonly screwed or bolted to each other to form the desired height assembly. Good fit and seal between the joints of the rings is desirable, with of course minimum labor of assembly. A riser should present an uppermost surface suitable for a lid with a good seal, particularly under conditions where surrounding soil may be prone to intruding into the seal region, as can occur when a lid is removed for septic tank maintenance purposes.
The present invention is also concerned with plastic tanks in the form of vertical cylinders for holding unpressurized water and other liquids. For example, a plastic tank may be used for storing water for domestic use. Distributors of tanks desire to provide users with different volume tanks. One way of achieving that is to form a tank from assembled components, similar to the way in which risers are made, and to provide a base closure.
Most commercially available risers are essentially short straight cylinders. Thus they cannot be conveniently nested for economic shipment and storage. Water tanks, for instance common bucket like tanks, often have tapered walls for easy nesting and shipment. However, it is an ongoing problem with respect to how to effective combine the principle of nesting tapered structures with the need for different length cylindrical structures which have substantially the same opening at the upper end as at the lower end, all the while getting good seals.